If this were mine, I'd go to my local bolt shop and buy 2 washers (recently purchased some 3" OD and they had larger on hand) with the OD of the indented flat area. Most likely the ID would have to the be drilled out larger to match the current opening for the shock. The idea would be to sandwich the flat area with the broken bolt tab between the 2 washers. Clamp the washers together and drill the 2 bolt holes for the shock mount. Weld 2 nuts to the top washer. Clean the broken flange really well. If I couldn't access the top of the arm to position the top washer I'd cut a slot in the bottom flange opposite the torn spot and slide the washer in from below. Drill a few hole in the bottom washer and arm flange to allow spot welds up to the top washer. Bolt the washers in place without the shock and spot weld it all together. Coat to prevent rust. Install shock and call it good.

Good luck!

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You can't always get what you want, but if you try sometime you just might find you get what you need.